Electrical – Modelling of a diode


so i have a SPICE simulation where i put a 1 volt sawtooth wave through a diode in the following circuit with the following graph

Circuit and resulting voltage/current characteristics

Then i can then plot the V-I graph, and then i've tried to use a curve fitting program in order to find the value of the saturation current, ideality factor and the other terms given by the diode equation,the problem is that the values don't make any sense as i get a value of 7.713E-08 for the saturation current and 0.15 for the ideality factor.
diode equation in curve fitting program
letting S be saturation current, and n being the ideality factor. thermal voltage is assumed to be 0.25
I've also tried the circuit without the resistor so the I-V graph takes into account the whole voltage drop of the circuit, but nothing seems to help. i've also tried using different diodes in SPICE(I'm using Partsim)

Am i doing something fundamentally wrong? and can this method be used in order to get the diode equation or not?


Best Answer

Jim's comment gets your main issue. A typical thermal voltage is 0.025 V, not 0.25 V. In any case, an error estimating the thermal voltage (or the junction temperature) will just cause an inversely proportional error in estimating the ideality factor.

The next issue is more subtle: To accurately estimate the saturation current, you'll want to measure the (very small) current in reverse bias.

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